In continuous casting a hot melt flows into a mould. In the mould the melt is cooled such that a solidified self-supporting surface layer is formed before the strand leaves the mould. If inflowing melt is allowed to flow into the mould in an uncontrolled manner, it will penetrate deep down into the non-solidified portions of the strand. This makes the separation of unwanted particles contained in the melt difficult. In addition, the self-supporting surface layer is weakened, which increases the risk of the melt breaking through the surface layer formed in the mould.
From, for example, SE-B-436 251, it is known to arrange one or more static or periodic low-frequency magnetic fields in the path of the melt to brake and distribute the inflowing melt.
The cast strand is formed by melt running down into the mould which is open downwards. The cast strand, which after the mould is to have a largely rectangular cross section, is formed by allowing the melt to flow into a tubular casting mould with a corresponding rectangular cross section, arranged in the mould. The walls of the casting mould consist of four separate copper plates. The copper plates are each fixed to a water box beam. The task of the water box beam is to stiffen the copper plate and, together with the copper plate, to enclose circulating cooling water.
When starting the casting operation, the mould is opened by hydraulic pistons pulling apart the copper plates and the associated water box beams such that a starting chain can be inserted between the copper plates. The mould is closed by the pistons pressing back the copper plates, which surround the starting chain.
The water box beams are surrounded by a retaining framework, to which the hydraulic pistons are attached. The water box beam with the copper plate constitute the movable side of the mould whereas the framework constitutes the fixed side.
According to patent application 9100184-2, the static or periodic low-frequency magnetic field is generated by means of magnetic field-generating devices which may consist of permanent magnets or coils, supplied with current, with magnetic cores. The magnetic field-generating devices will be referred to in the following as magnets.
The arrangement of the magnets in an existing machine for continuous casting will be described in the following.
The magnets have been arranged in the mould, between the water box beams and the framework. One magnet is placed on each side of the melt.
The water box beam cannot conduct the magnetic field since it consists for the most part of non-magnetic material. When the magnet is arranged between the water box beam and the framework, a longer core which reaches to the copper plate is therefore needed. The core is divided into a rear and a front core, and the front core has been integrated into the water box beam. In this way, the field is conducted through the water box beam.
After a relatively short time of use, the copper plates of the mould are in need of renovation, and then the whole mould is replaced by a renovated mould. Therefore, a plurality of moulds are associated with each continuous casting machine. During the renovation, the water box beam with the copper plate is removed from the mould and the copper plate is renovated. One of the reasons that the magnetic core is divided into a front and a rear part is to facilitate the removal of the water box beam during renovation of the copper plate.
To obtain a magnetic circuit, a magnetic return path is needed. The framework has been rebuilt and supplemented with more iron than what is justified from the point of view of strength, such that it can be utilized as a magnetic return path. The rear core is fixed to the framework. The framework and the cores together form a magnetic circuit.
The mould with magnets rests on a shaking table. To prevent the solidifying melt from adhering to the mould, an oscillating movement is imparted to the shaking table. An attachment device supports the mould and the shaking table. The attachment device does not oscillate along with the shaking table.
Since the rear core is fixed to the framework and the front to the water box beam, a problem arises in that an air gap is created between the movable and fixed parts when the mould is closed. When the mould is open, the air gap is closed. This air gap which separates the front and rear cores gives rise to an electromagnetic force which tends to close the air gap and hence open the mould during the casting. A known solution to this problem is to resist the electromagnetic force by means of hydraulic or mechanical pistons.
It is an object of the invention to suggest a continuous casting machine in which the magnetic field is returned without resulting in any annoying air gap.